1. Field of the Invention
The present invention relates to an apparatus for sensing the position of electrostatically driven XY-stage, and more particularly, to an apparatus for sensing the position of an electrostatic XY-stage through time-division multiplexing. The apparatus senses the positions of X- and Y-axes of the stage by applying an excitation signal having a phase difference to a stator of each axis of the XY-stage and then time-division detecting changes in capacitance in response to the signal.
2. Description of the Related Art
Recently, to overcome limits in the recording density of rotary magnetic recording media, high-density storage device using scanning probe microscopy (SPM) are being studied. The core technologies of high-density recording media relate to manufacture of a tip and a cantilever, recording and reproducing principles of the medium, signal processing, and manufacture of an XY-stage. In particular, the XY-stage and the signal processing circuitry take up a large portion of the storage device. Therefore, to reduce the overall size of a system, studies on increasing the driving force of the XY-stage per unit area or composing the signal processing circuitry of a signal chip are ongoing.
To detect the position of an XY-stage, the present invention employs capacitive sensing which is the principle of detecting the acceleration of a conventional electrostatic accelerometer. Since the principles of the present invention are applicable to the detection of acceleration, problems of a conventional method for detecting acceleration with an accelerometer will now be described.
One method for measuring acceleration in a multi-axis electrostatic accelerometer involves using accelerometers for each axis. However, the shortcomings of this method are that it is difficult to position accelerometers without interference between each axis and positioning accelerometers along each axis is costly. To surmount these shortcomings, a method of detecting acceleration along each axis by disposing proof masses sensitive to each axis on a silicon wafer and integrating detecting circuits of each axis has been proposed.
Furthermore, a method of detecting acceleration along multiple axes from a single proof mass has been proposed. The method is advantageous in terms of a small chip area and a low manufacturing cost. In the same context, according to a paper by M. A. Lemkin, B. E. Boser, D. Auslander and J. H. Smith, xe2x80x9cA 3-axis Force Balanced Accelerometer Using a Single Proof-massxe2x80x9d, International Conference on Solid-State Sensor and Actuators (TRANSDUCERS""97), Vol. 2, pp1185-1188, a carrier signal is applied to a proof mass and then the response to the signal is demodulated to measure changes in capacitance from a stator of each axis. Also, according to a paper by K. Jono, M. Hashimoto and M. Esashi, xe2x80x9cElectrostatic Servo System for Multi-axis Accelerometersxe2x80x9d, IEEE Workshop on Micro Electro Mechanical Systems(MEMS ""94) pp251-256, a carrier signal of different frequency is applied to a stator of each axis, and a synchronous demodulator of different frequency for each axis is connected to a proof mass, thus measuring acceleration along each axis. However, these methods have a drawback in that a chip area increases since a demodulating circuit is provided for each axis.
In a paper by H. Ahmad, A. J. Al-Khalili, L. M. Landsberger and M. Kabrizi, xe2x80x9cA two-dimensional micromachined accelerometerxe2x80x9d, IEEE Transactions on Instrumentation and Measurement, Vol. 46, No. 1, pp 18-26, 1999, interference between each axis is removed by switching a signal path by 12 switches, and changes in capacitance are measured in a time-division manner by a charge amplifier and a low-pass filter using a portion of the response to an excitation signal. However, this method has drawbacks in that many switches and circuits for processing signals on each axis are needed and that the amplitude of a signal is small and the signals are susceptible to noise since an average voltage for the cycle of an excitation signal is obtained by passing an output voltage of a pulse form which is proportional to changes in capacitance through a low-pass filter. U.S. Pat. No. 5,939,633 has addressed a method of detecting a change in capacitances by measuring voltages of a movable proof mass twice along each axis and calculating the difference between the measured voltages. However, the method has drawbacks in that the voltage of the proof mass is initialized at every cycle, interference between each axis exists unless a voltage of high frequency is applied, and a separate demodulator is used for the measurements along each axis. Furthermore, it has proven difficult to calculate acceleration from voltages of the proof mass measured twice with a time lag.
To solve the above problems, it is an objective of the present invention to provide an apparatus for sensing the position of an electrostatic XY-stage through time-division multiplexing, which is capable of sensing the positions of X- and Y-axes of the stage by applying an excitation signal having a phase difference to a stator of each axis of the XY-stage and then time-division detecting changes in capacitance from a moving plate.
Accordingly, to achieve the above objective, the present invention provides an apparatus for sensing the position of an electrostatic XY-stage through time-division multiplexing. The apparatus includes: a stage composed of a moving plate for moving a recording medium, rotor combs connected to the moving plate so that the rotor combs and the moving plate have the same electric potential, and stator combs of predetermined axes, wherein differential capacitors are formed between the rotor and stator combs; an amplifier connected to the stage for outputting position information in a time-division manner, in which a time constant is designed so as to prevent interference between position information output from the stage; a sample/hold unit for sampling and holding the time-division position information output from the amplifier in response to a timing control signal; and a controller which outputs a control signal such that the time-division position information from the amplifier can be sampled and held when the information reaches a maximum, reads position information output from the sample/hold unit, and generates excitation signals having different phases to each be applied to a different axis.